Formalize targets (users and channel) into objects

Summary:
Make users/channels/rooms into objects, so we can later sort out stuff like Campfire user IDs, Phabricator vs chat accounts, etc.

The only change here is that I removed output buffering from the macro handler. We should move throttling/buffering to adapters instead and have it apply globally.

Test Plan: Ran IRC and Campfire bots and interacted with them.

Reviewers: indiefan

Reviewed By: indiefan

CC: aran

Differential Revision: https://secure.phabricator.com/D4924